In 1952 the scientists at Porton Down were carrying out secret germ warfare trials off the coast of Lewis in the Outer Hebrides. One of the germs they were testing was plague -- the black death. Experimental animals kept in cages on a pontoon were infected with a plague cloud released upwind of them. One evening, as a trial was about to start, a trawler sailed unexpectedly into the danger zone. Rather than stop the tests, the captain of the naval ship gave the order to continue, and the germs were released into the path of the trawler. This caused the biological warfare specialists at Porton Down to tell the Admiralty that there was a risk of contamination. The matter was dealt with at the highest levels, with the involvement of the First Sea Lord and Rab Butler, who as Chancellor of the Exchequer was deputising for the absent Winston Churchill as Prime Minister. It was considered of the highest "political consideration" not to alert the crew and the nation to what had happened, not least because the scientists were warning that all the rats on the Carella should be killed and the ship fumigated and the Admiralty believed that would be a clear sign that plague was suspected. So the Carella and her crew of 18 were allowed to go back to port and to put to sea again. A destroyer and a fisheries vessel shadowed her from over the horizon, listening to her radio broadcasts, waiting to see if she called for medical assistance. Meanwhile plans were made for the immediate distribution of antibiotics along the North-West coast, should the Carell's crew show signs of plague. Jolyon Jenkins pieces together the events and the subsequent cover-up as the government tried to reconcile the conflicting demands of public health and official secrecy. The programme uses newly-declassified top secret documents, interviews with surviving witnesses, and the private diary of the chief scientist to shed light on a hitherto virtually unknown episode of the Cold War.